1. Field of the Invention
This invention relates to a process for manufacturing a multilayer rubber hose and a product thereof. More particularly, it relates to a simple and economical process which can manufacture a multilayer rubber hose having a high impermeability to fuel or a vaporized fuel from inexpensive rubber materials, and also to a product thereof.
2. Description of the Related Art
Regulations against the leakage of fuel from motor vehicles, which are similar to the SHED (Shield Housing for Evaporative Determination) in the United States, were put into force in Japan and Europe in 2000. As a result, there has been a strong demand for the improvement of fuel hoses which are highly responsible for the leakage of fuel from motor vehicles.
There is known a fuel hose having a two-layered wall formed of an inner layer of fluororubber (FKM) having a high fuel impermeability and an outer layer of a mixture of acrylonitrile-butadiene rubber and polyvinyl chloride (NBR-PVC), or epichlorohydrin rubber (ECO), as disclosed in Japanese Patent Application Laid-Open No. 171381/1992. Japanese Patent Application Laid-Open No. 304058/1999 discloses a fuel hose having a two-layered wall formed of an inner layer of a polyamide resin or fluororesin, and an outer layer of NBR-PVC or ECO. Japanese Patent Application Laid-Open No. 300392/1999 discloses a fuel hose having a three-layered wall formed of an inner layer of FKM or NBR, a middle layer of a fluororesin and an outer layer of NBR-PVC or ECO. These hoses, however, employ expensive materials, such as fluororubber, though they are improved in fuel impermeability. Moreover, the formation of a resin layer adds to the cost of manufacture.
Ordinary NBR type rubber, i.e. NBR or H-NBR or a mixture thereof with another kind of rubber, such as PVC, has been considered difficult to employ in view of the stringent regulations for fuel impermeability, though it may be inexpensive. It has, however, been found recently that NBR having a high AN content gives a high fuel impermeability. This fact indicates the possibility of making a fuel hose of high fuel impermeability from inexpensive NBR type rubber, as proposed in Japanese Patent Application Laid-Open No. 182450/1999 assigned to the same assignee as in this application.
NBR type rubber is, however, likely to have a low Mooney viscosity. The viscosity rapidly decreases particularly when it is heated for curing. It, therefore, follows that when a hose is made from NBR type rubber having a high AN content, it is likely to flatten except when it has a small inside diameter, or a large wall thickness relative to its inside diameter. In order to avoid such flattening, it is necessary to insert a mandrel into an extruded hose for curing. As is well known, the use of a mandrel for curing brings about a lower efficiency and a higher cost of production than when no mandrel is used. Thus, the known hoses of NBR type rubber having a high AN content have been unsatisfactory in cost reduction because of a still high cost of manufacture even though the cost of material may be lowered.
It is, therefore, an object of this invention to provide a process which can manufacture a fuel hose at a low cost from NBR type rubber having a high AN content. The inventors of this invention have found the following:
(1) The use of NBR type rubber having a high AN content for an inner wall layer enables the manufacture of a multilayer rubber hose of high fuel impermeability at a low cost;
(2) An outer wall layer can support the inner wall layer effectively against flattening if it is formed from an appropriate rubber composition of high flattening resistance, and extruded and cured simultaneously with the inner wall layer; and
(3) The rubber composition for the outer wall layer should be selected from among those having an appropriate minimum Mooney viscosity (JIS) at its curing temperature to ensure the formation of the outer wall layer supporting the inner wall layer effectively.
According to a first aspect of this invention, there is provided a process for manufacturing a rubber hose having a multilayer wall formed of at least an inner layer of rubber and an outer layer of rubber, the process comprising the steps of extruding the inner and outer layers simultaneously without the aid of any mandrel and curing them, wherein the hose has an inside diameter not exceeding 20 mm and a wall thickness of at least 2 mm, of which at least one-third is occupied by the thickness of the outer layer, and the inner layer is of a rubber composition containing NBR or H-NBR having a high AN content, while the outer layer is of a rubber composition having a minimum Mooney viscosity of 25 to 65 at its curing temperature.
The use of NBR type rubber having a high AN content for the inner wall layer enables the manufacture of a multilayer rubber hose having a high fuel impermeability at a low cost. A wall layer formed from NBR type rubber having a high AN content is, however, likely to flatten easily under heat. A rubber hose having a wall cured in a flattened state may have a warped cross section and its joint with a metal pipe or the like may not be tight enough to prevent any leakage of fuel.
According to the first aspect of this invention, therefore, the outer wall layer of the hose is of a rubber composition having a minimum Mooney viscosity of 25 to 65 at a curing temperature, and the inner and outer wall layers are extruded and cured together, so that the outer layer may support the inner layer effectively to prevent its flattening. The inside diameter and the wall thicknesses of the hose and the thickness of the outer layer as stated above ensure that no flattening of its inner layer occur even if it may be extruded and cured without the aid of any mandrel. The process not relying upon any mandrel enables a corresponding reduction in the cost of manufacture.
The outer layer may fail to support the inner layer effectively if the hose does not satisfy any of the diameter and thickness requirements stated above. However, a hose having an inside diameter of less than 3 mm or a wall thickness over 5 mm may not be capable of conveying fuel at a satisfactorily high flow rate and may not be suitable for practical use. Moreover, any outer layer having a thickness exceeding {fraction (9/10)} of the whole thickness of the wall may make the inner layer unsatisfactory in fuel impermeability.
According to a second aspect of this invention, the rubber composition forming the inner layer comprises NBR or H-NBR having an AN content of 43 to 55%, or a mixture thereof with PVC. The inner layer of the composition as defined has a particularly high fuel impermeability. Its minimum Mooney viscosity at a curing temperature is, however, so low that the outer layer may provide a particularly effective support therefor. The mixture containing PVC is, among others, preferred for its high fuel impermeability, and also as it imparts weatherability and ozone resistance to the rubber composition.
According to a third aspect of this invention, the rubber composition forming the outer layer is selected from among NBR-PVC, NBR-EPDM (a mixture of acrylonitrile-butadiene rubber and an ethylene-propylene-diene terpolymer), hydrin rubber, CPE (chloropolyethylene rubber), CR (chloroprene rubber) and CSM (chlorosulfonated polyethylene rubber) These materials are all high in weatherability and ozone, fuel and oil resistance, and available at a low cost. In this aspect, the NBR type rubber for the outer layer does not have a particularly high AN content.
According to a fourth aspect of this invention, the rubber composition forming the outer layer is a mixture containing 55 to 80% by weight of NBR having an AN content of 25 to 43% and 20 to 45% by weight of PVC. This composition is preferred for ensuring easily a minimum Mooney viscosity of 25 to 65 at a curing temperature. Moreover, it has a certain level of fuel impermeability adding to that of the inner layer, while it is high in weatherability and ozone, fuel and oil resistance.
According to a fifth aspect of this invention, the rubber composition forming the outer layer is a mixture containing 30 to 70% by weight of NBR having an AN content of 25 to 43% and 30 to 70% by weight of EPDM. This composition is preferred for the same reasons as the mixture according to the fourth aspect of this invention is. It easily ensures a minimum Mooney viscosity of 25 to 65 at a curing temperature and has a certain level of fuel impermeability adding to that of the inner layer, while it is high in weatherability and ozone, fuel and oil resistance.
According to a sixth aspect of this invention, the mixture of NBR and PVC, or NBR and EPDM further contains 10 to 40 phr (parts per hundred parts of rubber) of a plasticizer, 40 to 120 phr of carbon black having a DBP (dibutyl phthalate) absorption of at least 100 ml/100 g, and/or 15 to 70 phr of a silica type filler. The addition of a plasticizer is useful for adjusting the minimum Mooney viscosity of the composition, raising the flexibility of the outer layer and improving the characteristics of the hose at a low temperature. The addition of carbon black is useful for adjusting the minimum Mooney viscosity of the composition and raising the electrical conductivity of the hose to prevent any trouble caused by its electrical charging. The addition of a silica type filler is useful for adjusting the minimum Mooney viscosity of the composition and improving the resistance of the hose to aging under heat and deterioration by a fuel oil.
According to a seventh aspect of this invention, there is provided a rubber hose manufactured by the process according to any of the first to sixth aspects of this invention. The hose is inexpensive to manufacture, has a high fuel impermeability and is free from any defect, such as a defective appearance due to the flattening of its wall under heat at a curing temperature, or a defective seal in its joint with a metal pipe.
According to an eighth aspect of this invention, the hose is a fuel hose for a motor vehicle. It is particularly suitable as a fuel hose for a motor vehicle, though it may be useful for any other purpose.